A number of processes are known for the conversion of synthesis gas (further referred to herein as syngas) to a mixture of normally gaseous and normally liquid and optionally normally solid hydrocarbons. An example of such a process is the Fischer Tropsch synthesis process. In this process, the syngas is usually converted over a suitable catalyst at elevated temperature and pressure into a mixture of hydrocarbon compounds, and water. The reaction affords mainly aliphatic straight-chain hydrocarbons, more specifically mostly into paraffinic compounds ranging from methane to high molecular weight molecules comprising up to 200 carbon atoms, but also branched hydrocarbons, unsaturated hydrocarbons, and primary alcohols are formed. Numerous catalysts are known for the Fischer Tropsch process, mainly based on iron or cobalt. Iron-based catalysts usually have a high tolerance for sulphur, are relatively cheap, and produce a mixture of saturated hydrocarbons, olefins, and alcohols. Cobalt-based catalysts usually provide for a higher conversion rate, and are more reactive for hydrogenation and produce therefore less unsaturated hydrocarbons and alcohols compared to iron-based catalysts.
Independently from the exact nature of the catalyst employed, its activity and selectivity for heavier hydrocarbon products degenerates successively during operation. Once a catalyst has been deactivated below a certain activity and/or selectivity level, this spent catalyst needs to be reconditioned or exchanged after a certain time of operation. The active lifetime of the catalysts thus is determined by the overall reaction rate that is achieved in a reactor unit, and can be as limited as several weeks of continuous operation in commercial installations in the case of iron-based catalysts.
Usually, increasing the temperature of the reactor in question compensates the reduction in catalyst activity. However, by raising the temperature with increasing catalyst deactivation, the amount of smaller hydrocarbon products produced increases, thus reducing the overall quality of the product mixture. For a given catalyst and syngas composition the selectivity to liquid products (as expressed by the selectivity to product having more than 5 carbon atoms in a chain, further referred to as C5+) decreases with decreasing activity (factor) when compensated by a higher temperature. Moreover, selectivity for CO2 formation increases, thereby further decreasing the production of valuable C5+ products, while equally reducing the efficiency of the process.
Once a catalyst has reached a certain level of deactivation, the catalyst deactivation accelerates under a constant syngas flow, and critical values may be reached requiring rapid replacement. The catalyst reactivation again often requires removal of the catalyst from the reactor vessel, thereby involving cumbersome handling of catalyst material.
The catalyst deactivation also reduces the obtainable steam temperature and pressure, and may lead to steam quality issues since the lower reaction rate leads to a lower amount of heat generated for superheating steam. Accordingly, the process according to the present invention provides for an improved process for the conversion of a gaseous mixture of carbon monoxide and hydrogen to a mixture of normally gaseous and normally liquid and optionally normally solid hydrocarbons and water, wherein the above identified disadvantages are at least reduced. It further provides for a reactor line-up that allows operating the process under optimal conditions and increased C5+ selectivity with decreased CO2 production.